Expose a module's function names in modulemeta

[mrwilson]

This PR adds a new field, defs, to the output of modulemeta.

This field contains the names and arities of functions defined within that module. For example

module {
  name: "example",
  version: "0.0.0"
};

def foo(): 1;
def bar($baz): $baz + 1;

$ jq -L. --null-input '"example" | modulemeta | .defs[]'

"foo/0"
"bar/1"

The intent of this change is to make it easier for tools / processes built on top of jq to introspect about function definitions within the modules.

[nicowilliams]

Do we want to also list the module's $bindings, if any?

[nicowilliams]

Thanks!